WASHINGTON —The Department of Homeland Security has issued an alert to airlines about a possible shoe bomb threat on flights entering the U.S. from abroad, a department official said.

The bulletin, which did not outline a specific plot, is not related to an earlier alert linked to the Winter Olympic Games involving possible devices concealed in toothpaste tubes.

"Out of an abundance of caution, DHS regularly shares relevant information with domestic and international partners about relevant threat information as we work to meet our mission of keeping the traveling public safe,'' said the DHS official, who is not authorized to comment publicly. "These types of regular communications are part of that important priority.''

A shoe bomb was one of the reasons for tightening airline security after the terrorist hijackings Sept. 11, 2001. A passenger, Richard Reid, tried to detonate explosives in his shoes on a flight from Paris to Miami in December 2001. But other passengers subdued him and he is serving a life sentence in prison.